DNA damage is clearly a threat to the integrity of the genome and consequently cells have evolved mechanisms (termed checkpoints) that respond to damage and temporarily arrest the cell cycle to allow time for repair. In Saccharomyces cerevisiae, DNA damage blocks the progression through the cell cycle at the G1/S, intra-S, and G2/M transitions. Our preliminary studies have uncovered a fourth DNA damage checkpoint response that works in late anaphase. In budding yeast, the RAD9, RAD17, RAD24, RAD53 (MEC2). MEC1 and MEC3 gene products are required for the DNA damage checkpoint response but their mechanism of action is still unclear. Significant progress has been made defining the biochemical properties of and physical interactions between the various checkpoint proteins; however, the functional consequences of these interactions remains unclear. Taking a genetic and biochemical appraoch, we propose to study the targets of the DNA damage signal transduction cascade that cause cell cycle arrest in metaphase and late anaphase. Our work primarily focuses on Pds1, Cdc20, and other proteins that are potential effectors of the DNA damage checkpoint. To help us understand why Pds1 is required for the damage response, we plan to study the role of Pds1 in normal cell cycle progression. Our preliminary results suggest that Pds1 not only inhibits the metaphase to anaphase transition, as documented, but also plays a normal role in inhibiting cytokinesis. If Pds1 plays a role in controlling the onset of anaphase and cytokinesis, degradation of it may be the initiating event that leads to mitotic kinase inactivation and cell cycle exit. Understanding this process will shed light on how the DNA damage checkpoint can work in late anaphase to prevent mitotic exit in response to irradiation. Our ultimate goal is to identify the complex network of proteins involved in eliciting damage-induced cell cycle arrests and understand their role in normal cell cycle progression. Increasing our knowledge of how the cell cycle is controlled will help us understand why it becomes unregulated in cancer cells, thereby allowing us to develop new methods for reversing the transformed phenotype.